Process for producing steel for high-carbon steel wire material with excellent drawability and fatique characteristics

ABSTRACT

The present invention provides a method suitable for manufacturing a steel material for obtaining a steel wire rod with decreased amount of hard nonmetallic inclusions and improved drawability and fatigue property by adequately controlling the conditions of secondary refining and manufacturing conditions in a converter. Converter blowing is performed by taking molten iron, cold iron, and steel scrap as main raw materials to be charged into a converter, the ratio of these components based on all the main raw materials being such that the molten iron takes 96 to 100% (means wt. %, same hereinbelow), the cold iron takes 4% or less, and the steel scrap takes 2% or less, and by setting an average P concentration in all the main raw materials to 0.02% or less, and operations are carried out such that a flow rate of gas for stirring molten steel during secondary refining after completion of the converter blowing is set to 0.0005 Nm 3 /min or more to 0.004 Nm 3 /min or less per 1 t of molten steel, and then a flow rate of Ar used to purge the inside of a tundish in continuous casting is set to 0.04 Nm 3 /min or more to 0.10 Nm 3 /min or less per 1 t of molten steel within the tundish.

FIELD OF THE INVENTION

The present invention relates to a method for manufacturing a steel material for obtaining a high-carbon steel wire rod with excellent drawability and fatigue property, and more particularly to a method suitable for manufacturing a steel for a steel wire rod in which the amount of nonmetallic inclusions having high hardness and very low ductility is reduced and the drawability and fatigue property are improved.

BACKGROUND ART

Where nonmetallic inclusions (in particular oxide inclusions; referred to hereinbelow simply as “inclusions”) that are hard and have a very low ductility, such as alumina (Al₂O₃) or spinel (Al₂O₃.MgO) are contained in a steel for tire cords or a steel for springs, the inclusions cause a loss of drawability in the process of drawing into an ultrafine steel wire and become the starting points for fatigue fracture at the product stage. Therefore, it is important to reduce the content of inclusions to a minimum or increase the ductility thereof by softening, thereby rendering them harmless, in the process of manufacturing the steel wire rod.

A variety of methods have heretofore been suggested from a view point of reducing to a minimum the amount of impurities present in steel wire rods. For example, Patent Documents 1, 2 disclose a method for reducing the amount of impurities by using Si and Mn as deoxidizing agents for a steel melt and regulating the concentration of Al. Furthermore, Patent References 3, 4 suggest a technology relating to the reduction in the amount of impurities by regulating the concentration of Al₂O₃ present in a refractory container that accommodates the steel melt. A technology for reducing the amount of impurities in steels by refining the steel melt by using a CaO—SiO₂ flux with a low concentration of Al₂O₃ has also been suggested (for example, Patent Documents 5, 6).

However, the technologies that have been heretofore suggested relate to a secondary refining process performed with respect to a molten steel tapped from a converter, and even if the conditions of such secondary refining process are adequately controlled, the amount of impurities cannot be sufficiently reduced. Accordingly, it is necessary to control adequately the conditions of both the secondary refining and the processes preceding the secondary refining.

[Patent Document 1] Japanese Patent Application Laid-open No. 50-081907, Claims, etc. [Patent Document 2] Japanese Patent Application Laid-open No. 50-11618, Claims, etc. [Patent Document 3] Japanese Patent Application Laid-open No. 2003-245758, Claims, etc. [Patent Document 4] Japanese Patent Application Laid-open No. 2004-211148, Claims, etc. [Patent Document 5] Japanese Patent Application Laid-open No. 4-110413, Claims, etc. [Patent Document 6] Japanese Patent Application Laid-open No. 9-059744, Claims, etc. DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

The present invention was made in view of above circumstances, it is an object of the present invention to provide a suitable method for manufacturing a steel material for obtaining a steel wire rod that has improved drawability and fatigue property and in which the amount of hard nonmetallic inclusions is reduced by adequately controlling the conditions in secondary refining and manufacturing conditions in a converter.

Means for Resolving the Problems

The essence of the manufacturing method in accordance with the present invention that successfully attains the above-described object is that converter blowing is performed by taking molten iron, cold iron, and steel scrap as main raw materials to be charged into a converter, the ratio of these components based on all the main raw materials being such that molten iron takes 96 to 100% (means wt. %, same hereinbelow), cold iron takes 4% or less, and steel scrap takes 2% or less, and by setting an average P concentration in all the main raw materials to 0.02% or less, and that operations are carried out such that a flow rate of gas for stirring molten steel during secondary refining after completion of the converter blowing is set to 0.0005 Nm³/min (N means “normal” and represents a volume at 298 K and 10⁵ Pa; same hereinbelow) or more to 0.004 Nm³/min or less per 1 t of molten steel, and then a flow rate of Ar used to purge the inside of a tundish in continuous casting is set to 0.04 Nm³/min or more to 0.10 Nm³/min or less per 1 t of molten steel within the tundish.

The steel for a steel wire rod that is an object of the method in accordance with the present invention preferably has a chemical composition comprising C, 0.4 to 1.3%, Si: 0.1 to 2.5%, Mn: 0.2 to 1.0%, and Al: 0.003% or less with Fe and unavoidable impurities as the balance.

If necessary, the steel for a steel wire rod that is an object of the method in accordance with the present invention also can contain as another element at least one element selected from the group consisting of (a) Ni: 0.05 to 1%, Cu: 0.05 to 1%, and Cr: 0.05 to 1.5% and at least one element selected from the group consisting of (b) Li: 0.02 to 20 ppm, Mg: 0.02 to 20 ppm, Ce: 3 to 100 ppm, and La: 3 to 100 ppm, and the properties of the steel wire rod can thus be further modified according to the type of the contained components.

EFFECT OF THE INVENTION

In accordance with the present invention, by adequately controlling the ratio of raw materials and an average P concentration in the main raw materials to be charged into a converter and also adequately controlling the flow rate of a stirring gas of molten steel in secondary refining and a flow rate of Ar used for purging the inside of the tundish during continuous casting, it is possible to reduce the amount of hard nonmetallic inclusions and obtain a steel for a steel wire rod that has excellent fatigue property and to provide with good efficiency a steel for a steel wire rod that is optimum for the manufacture of high-strength ultrafine wires for tire cords and springs that are required to have good fatigue property.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating the relationship between the molten iron ratio in the main raw materials and the number of wire breakages per 10 t of steel.

FIG. 2 is a graph illustrating the relationship between the cold iron ratio in the main raw materials and the number of wire breakages per 10 t of steel.

FIG. 3 is a graph illustrating the relationship between the steel scrap ratio in the main raw materials and the number of wire breakages per 10 t of steel.

FIG. 4 is a graph illustrating the relationship between the flow rate of purge Ar in a tundish and the number of wire breakages per 10 t of steel.

BEST MODE FOR CARRYING OUT THE INVENTION

In converter operations, steel scrap from inside and outside the steelmaking plant or cold iron, which is a solidified molten iron, is charged in addition to molten iron manufactured in a blast furnace, the molten iron temperature is raised, while oxidizing and removing C, and a molten steel with a C concentration of about 0.03 to 1% is manufactured. The temperature of the molten iron that is charged into the converter in this process is about 1200 to 1400° C. The higher is this temperature, the larger amount of steel scrap or cold iron can be charged into the converter, thereby making it possible to manufacture a large amount of molten steel by using a small amount of molten iron.

However, when high-carbon steel such as steel for tire cord or steel for springs is manufactured, P is difficult to remove in a converter, without decreasing the production efficiency. Therefore, it is necessary to reduce as much as possible the concentration of P in the main raw materials such as molted iron, cold iron, and steel scrap that will be charged into the converter. Examples of other raw materials that are charged into the converter in addition to the molten iron, cold iron, and steel scrap include an iron ore for adjusting slag formation and dolomite for protecting the converter refractory, but in the present invention, raw materials (molten iron, cold iron, and steel scrap) that do not include these additional components are referred to as the main raw materials.

From amongst the main raw materials, molten iron is subjected to dephosphorization treatment prior to charging into a converter. Therefore, the temperature of the molten iron charged into the converter is low and a thermal margin necessary for charging the steel scrap is small. The low thermal margin can be compensated by excess oxidation of C contained in the molten iron, but in the manufacture of steel for tire cords or steel for springs that are adversely affected by hard inclusions such as alumina every effort has to be made to avoid the excess oxidation of C. Thus, the excess oxidation of C increases the concentration of dissolved oxygen in molten steel at the completion of converter blowing and increases the amount of FeSi alloy used in deoxidation of molten steel. Furthermore, a small amount of Al is unavoidably contained in the FeSi alloy and, as a result, the amount of alumina-based inclusions in the molten steel is increased.

Accordingly, in order to reduce the content of alumina-based inclusions in molten steel, it is necessary to maximize the thermal margin of the main raw materials that are charged into a converter. The investigation conducted by the inventors with this consideration in view demonstrated that adequately selecting the ratio of molten iron, cold iron, and steel scrap in the main raw materials is effective in terms of increasing the thermal margin of the main raw materials.

First, the inventors examines the relationship between the molten iron ratio, cold iron ratio, and steel scrap ratio in the main raw materials (the ratio of each raw material when the sum total of the molten iron, cold iron, and steel scrap is taken as 100%) that will be charged into a converter and the number of wire breakages during drawing of 10 t of steel wire rods. The relationship between the molten iron ratio in the main raw materials and the number of wire breakages is shown in FIG. 1, the relationship between the cold iron ratio in the main raw materials and the number of wire breakages is shown in FIG. 2, and the relationship between the steel scrap ratio in the main raw materials and the number of wire breakages is shown in FIG. 3 (the meaning of the number of wire breakages is explained in the below-described embodiments).

The results clearly indicate that the number of wire breakages increases when the molten iron ratio is less than 96%, cold iron ratio is more than 4%, or steel scrap ratio is more than 2%. Therefore, it is clear that using main raw materials for charging into a converter that comprise molten iron at a ratio of 96% or more, cold iron at a ratio of 4% or less, and steel scrap at a ratio of 2% or less is effective in terms of reducing the number of breakages. The molten iron ratio is preferably 98% or more, even more preferably 100%. The preferred range of the cold iron ratio is 2% or less.

In accordance with the present invention, the average P concentration in the raw materials charged into a converter has to be 0.02% or less. Dephosphorization during converter blowing proceeds simultaneously with decarburization, but because in high-carbon steels such as steels for tire cords and steels for springs, decarburization has to be suppressed, dephosphorization cannot be expected to proceed during converter blowing. Further, if the concentration of P in the steel wire rod exceeds 0.02%, the segregation of P causes more frequent occurrence of wire breakage and decrease in fatigue strength. Accordingly the concentration of P in the entire raw material has to be decreased to 0.02% or less before the converter blowing. It is preferred that this concentration be 0.015% or less.

In accordance with the present invention, the stirring gas flow rate (sometimes referred to hereinbelow simply as “gas flow rate”) during stirring of slag and molten steel also has to be adequately controlled during secondary refining (for example, ladle refining) after completion of converter blowing. Molten steel after completion of converter blowing is deoxidized with Si or Mn, but in this process Al₂O₃ is generated by a very small amount of Al contained in the alloy (deoxidizing agent) such as FeSi, FeMn, and SiMn, and harmful inclusions remain in the product. Accordingly, Al₂O₃ has to be removed during secondary refining.

From this standpoint, it is necessary to intensify the contact between slag and molten steel by increasing the gas flow rate to 0.0005 Nm³/min or more per 1 t (ton) of molten steel. The gas flow rate is preferably 0.0006 Nm³/min or more, more preferably 0.0007 Nm³/min or more. However, if the gas flow rate is too high, loss of ladle refractories on melting becomes significant and undesirable in terms of operation. Moreover, the refractories are admixed to the molten steel, thereby adversely affecting the product. Accordingly, the gas flow rate has to be 0.004 Nm³/min or less, preferably 0.0035 Nm³/min or less, more preferably 0.003 Nm³/min or less.

No specific limitation is placed on the gas to be used for the stirring, and argon, which does not react with molten steel and is available at a low cost, is a suitable gas. No specific limitation is also placed on the gas blowing method, and a method for blowing through a refractory nozzle from above the molten steel or a method for blowing from the bottom or side surface of the ladle can be employed.

The molten steel subjected to secondary refining is cast in a continuous casting machine. In continuous casting, the molten steel is transferred and temporarily accommodated in a container called “tundish”. If air remains in the space inside the tundish, then molten steel is oxidized by oxygen present in the air, thereby generating inclusions and increasing the breaking frequency of wire when a steel wire rod is produced. Accordingly, the inside of the tundish has to be purged during casting with Ar gas.

FIG. 4 shows the relationship between the purge Ar flow rate in the tundish (flow rate per 1 t of molten steel inside the tundish) and the number of wire breakages (per 1 t of molten steel). The results clearly indicate that when the purge Ar flow rate is less than 0.04 Nm³/min, the oxidation of molten steel by oxygen contained in the air becomes significant and the number of wire breakages increases. Accordingly, the purge Ar flow rate has to be 0.04 Nm³/min or more. However, if the purge Ar flow rate per 1 t of molten steel exceeds 0.10 Nm³/min, the aforementioned effect tends to be saturated.

In the steel material (high-carbon steel) that is the object of the present invention, only the amount of Al from amongst the steel components is limited as described hereinbelow, no special limitation is placed on other components and, as shown hereinabove, they are contained in the same amounts as in generally used steel for springs or steel materials for drawing such as steel cords. More specifically, the steel contains C, 0.4 to 1.3%, Si: 0.1 to 2.5%, Mn: 0.2 to 1.0%, and Al: 0.003% or less (excluding 0%). Reasons for setting such preferred ranges of components are described below.

[C: 0.4 to 1.3%]

C is an element useful for increasing the strength and is preferably contained at 0.4% or more to demonstrate this effect. Even more preferred content is 0.5% or more. However, if the content of C becomes too high, the steel is embrittled and loses its drawability. Therefore, it is preferred that the content of carbon be suppressed to 1.3% or less (more preferably, 1.2% or less).

[Si: 0.1 to 2.5%]

Si is an element demonstrating a deoxidizing function. For this function to be demonstrated, it is preferred that the content of silicon be 0.1% or more, even more preferably 0.2% or more. However, if the content of Si is too high, a large amount of SiO₂ is generated as a deoxidization product and the steel loses its drawability. Therefore, the content of Si is preferably suppressed to 2.5% or less (more preferably 2.3% or less).

[Mn: 0.2 to 1.0%]

Mn is an element that demonstrates a deoxidizing function, similarly to Si, and also has a function for controlling inclusion property. For this functions to be demonstrated effectively, it is preferred that the content of Mn be 0.2% or more (more preferably 0.3% or more). On the other hand, if the amount of Mn is too high, the steel is embrittled and loses its drawability. Therefore, it is preferred that the content of manganese be suppressed to 1.0% or less (more preferably, 0.9% or less).

[Al: 0.003% or Less]

If the content of Al increases, the concentration of Al₂O₃ in inclusions rises, and large-sized Al₂O₃ that causes breakage of wire can be formed. Therefore, the content of aluminum is preferably suppressed to a minimum. From this standpoint, it is preferred that the content of aluminum be suppressed to 0.003% or less (more preferably 0.002% or less).

The basic components of the steel material that is the object of the present invention are described above, the balance being iron and unavoidable impurities. As for impurities, the admixture of elements carried by raw materials, source materials, and production equipment can be allowed. Furthermore, properties can be further effectively improved by introducing the following elements.

[At Least One Element Selected from the Group Consisting of Ni: 0.01 to 1%, Cu: 0.01 to 1%, and Cr: 0.05 to 1.5%]

Ni is an element making no significant contribution to the increase in strength of steel wire, but demonstrating an effect of increasing the toughness of drawn wire material. For this effect to be demonstrated, it is preferred that the content of nickel be 0.01% or more, more preferably 0.02% or more. However, if the content of Ni is too high, the aforementioned effect reaches saturation. Therefore, it is preferred that the content of nickel be 1% or less (more preferably 0.9% or less).

Cu is an element making contribution to strengthening of steel wire due to precipitation hardening function thereof. To demonstrate such an effect, the contents of copper is preferably 0.01% or more, more preferably 0.02% or more. However, if the content of Cu is too high, it precipitates on crystal grain boundaries, and cracks or scratches easily occur in the process of hot rolling the steel wire. Therefore, the content of copper is preferably 1% or less (more preferably, 0.9% or less).

Cr increases a work hardening ratio during wire drawing and easily ensures a high strength even at a comparatively low processing ratio. Moreover, Cr also acts to increase the corrosion resistance of steel. For example, when the wire is to be used as a reinforcing material for rubber (ultrafine steel wire), e.g. in tires, chromium effectively acts to inhibit the corrosion of the ultrafine steel wire. For this effect to be demonstrated, it is preferred that the content of Cr be 0.05% or more, more preferably 0.1% or more. However, if the amount of Cr is too high, hardenability with respect to pearlitic transformation increases and the patenting treatment is made difficult. Furthermore, the amount of secondary scale and density thereof increase, and the mechanical descaling ability and pickling ability are degraded. Therefore, the content of Cr is preferably 1.5% or less, more preferably 1.4% or less.

[At Least One Element Selected from the Group Consisting of Li: 0.02 to 20 ppm, Mg: 0.02 to 20 ppm, Ce 3 to 100 ppm and La: 3 to 100 ppm]

These elements act to further soften the nonmetallic inclusions present in steep. For this effect to be demonstrated, it is preferred that in the case of Li the content be 0.02 ppm or more (more preferably 0.03 ppm or more), in the case of Mg the content be 0.02 ppm or more (more preferably 0.03 ppm or more), in the case of Ce the content be 3 ppm or more (more preferably 5 ppm or more), and in the case of La the content be 3 ppm or more (more preferably 5 ppm or more). However, because the effect reaches saturation even if these elements are introduced in excess, the amount of Li and Na may be restricted to 20 ppm or less (more preferably 10 ppm or less) each. Further, the amount of Ce and La may be restricted to 100 ppm or less (more preferably 80 ppm or less) each.

The steel material obtained by the manufacturing method in accordance with the present invention is thereafter hot rolled to obtain a steel wire rod. The cross section diameter of the wire rod is 3 to 10 mm. This steel wire rod is suitable as a workpiece for ultrafine high-strength steel wire such as a tire cord or a piano wire that is required to have high drawability, for example, in a cold drawing process. Further, the steel wire rod is also suitable as a workpiece for springs or wires that require good fatigue property.

The present invention will be described below in greater detail with reference to embodiments thereof, but these embodiments place no limitation on the present invention, and the invention can be implemented by introducing appropriate changes within a range that can conform to the above- and below-described essence of the invention.

EMBODIMENTS Manufacture of Steel Material

Molten iron in which the content of P and S was decreased to 0.007 to 0.020% and 0.002 to 0.01%, respectively, in the preliminary treatment process of the molten iron, or such molten iron mixed at various ratios with cold iron and/or steel scrap was charged into a converter and decarburization blowing was performed to a predetermined C concentration. Then, the steel was tapped into a ladle, and composition adjustment (the composition is presented in Tables 2, 5 below) and slag refining (secondary refining) were preformed in a ladle furnace. The slag during ladle refining was of a CaO—SiO₂—Al₂O₃ system with a CaO/SiO₂=0.7 to 1.7 and Al₂O₃=4 to 25%. Further, Ar was used as a stirring gas for molten steel during ladle refining, and the flow rate of argon was changed within a range of 0.0002 to 0.0080 Nm³/min/t per 1 t of molten steel. The gas stirring time was 15 min or more in all the cases.

Continuous casting was conducted following the ladle refining and a bloom with a cross section of 600 mm×380 mm was obtained. The amount of molten steel in a tundish during casting was 20 t, and the purge Ar flow rate was changed within a range of 0.02 to 0.13 Nm³/min/t per 1 t of molten steel. The bloom was then heated to 1260° C. and subjected to billetting to obtain a square cross section with a size of 155 mm. Then, hot rolling was performed to obtain a steel wire rod with a diameter of 5.5 mm or 8.0 mm.

A piece of 1000 g was cut from the obtained steel wire rod and used for inclusion extraction by acid dissolution and composition analysis of inclusions. Methods for the inclusion extraction and composition analysis (quantities) of inclusions are described below.

[Method for Inclusion Extraction]

First, a beaker was prepared containing an acid solution in which pure water, nitric acid (concentration 60%), and sulfuric acid (concentration 96%) were mixed at a volume ratio of 5:25:1, respectively, and the steel wire rod (1000 g) was placed into the beaker. The beaker was heated, and the wire rod was completely dissolved, while maintaining the solution temperature at 90 to 95° C. Upon dissolution, filtration with a 10 μm filter was performed. The composition of inclusions with a long diameter of 20 μm or more, from amongst the inclusions remaining in the filter, was analyzed and the number of such inclusions was measured.

[Quantitative Determination of Inclusions]

An EPMA [Electron Probe Microanalyzer, manufactured by Japan Electron Optics Laboratory Co., Ltd. (JXA-8000 Series)] was used for quantitative determination of inclusions, and quantitative analysis was conducted by characteristic X-ray energy dispersion spectroscopy under the conditions of accelerating voltage 20 kV and probe current 0.01 μA. Elements that were the objects of the quantitative analysis included Al, Mn, Si, Mg, Ca, Ti, Zr, and O. The method for quantitative determination included the steps of measuring X-ray intensity of a substance of already known concentration of these elements, plotting the relationship between the X-ray intensity and element concentration in advance as a calibration curve, and finding the concentration at which each element is present from the X-ray intensity of the inclusions that are the observation objects by using the calibration curve. Each element was assumed to be present in the form of Al₂O₃, MnO, SiO₂, MgO, CaO, TiO₂, ZrO₂, the concentration at which the Al₂O₃, MnO, SiO₂, MgO, CaO, TiO₂, ZrO₂ were present in the inclusions was calculated based on the concentration of each element found by the above-described quantitative analysis, inclusions containing 80% or more of Al₂O₃ were taken as alumina-based inclusions, and the long diameter and number thereof were measured.

Embodiment 1 Evaluation of Drawability

Drawability in the case of applying the steel wire rod with a diameter of 5.5 mm that was obtained in the above-described manner to a tire cord was evaluated according to the following items.

(Evaluation Method)

Number of wire breakages during drawing from a diameter of 5.5 mm to a diameter of 0.2 mm.

(Drawing Method)

An oxidation surface film on the steel wire rod with a diameter of 5.5 mm was removed with hydrochloric acid and then dry drawing was conducted to a diameter of 1.2 mm with a continuous drawing machine (model CD-610−7+BD610 manufactured by Showa Machine Works, Ltd.). The diameter of drawing dies used in the drawing process was 4.8, 4.2, 3.7, 3.26, 2.85, 2.5, 2.2, 1.93, 1.69, 1.48, 1.3 (units: mm for all the dies). The drawing speed in the die with a diameter of 1.2 mm was 400 m/min. The wire rod surface was coated with zinc phosphate prior to drawing, and the drawing was preformed by using a lubricant based on sodium stearate.

The wire rod that was drawn to a diameter of 1.2 mm was heated to 1230 K, and subjected to patenting in a lead bath at 830 K to obtain a fine pearlitic structure, followed by plating with brass (film thickness: about 1.5 um) containing Cu and Zn at a 7:3 ratio (mass ratio). Finally, the wire rod was drawn to the diameter of 0.2 mm with a wet-type drawing machine (Type KPZIII/25-SPZ250, manufactured by Koch, Ernst & Co., Ltd.). In the dipping bath employed for wire drawing, a solution containing 75% water and prepared by mixing a natural fatty acid, an amino acid, and a surfactant was used. The diameter of the dies used in the drawing process was 1.176, 0.959, 0.880, 0.806, 0.741, 0.680, 0.625, 0.574, 0.527, 0.484, 0.444, 0.408, 0.374, 0.343, 0.313, 0.287, 0.260, 0.237, and 0.216 (units: mm for all the dies). The drawing speed at a diameter of 0.2 mm was 500 m/min.

The conditions relating to the main raw materials for a converter are shown in Table 1 below, the chemical compositions of the steel wire rods are shown in Table 2 below, and the results on drawability, together with the conditions of secondary refining, are shown in Table 3 below.

TABLE 1 Raw materials for converter Molten Cold Molten Cold iron Scrap P Test iron iron Scrap Total iron ratio ratio ratio concentration No. (t) (t) (t) (t) (wt. %) (wt. %) (wt. %) (wt. %) 1 264 0 0 264 100.0 0.0 0.0 0.007 2 260 0 0 260 100.0 0.0 0.0 0.010 3 260 0 2 262 99.2 0.0 0.8 0.015 4 257 0 5 262 98.1 0.0 1.9 0.011 5 252 5 3 260 96.9 1.9 1.2 0.013 6 252 5 5 262 96.2 1.9 1.9 0.013 7 252 5 0 257 98.1 1.9 0.0 0.014 8 250 10 0 260 96.2 3.8 0.0 0.017 9 252 9 0 261 96.6 3.4 0.0 0.022 10 258 0 4 262 98.5 0.0 1.5 0.025 11 264 0 0 264 100.0 0.0 0.0 0.007 12 260 0 4 264 98.5 0.0 1.5 0.010 13 260 4 2 266 97.7 1.5 0.8 0.012 14 254 9 0 263 96.6 3.4 0.0 0.009 15 255 0 0 255 100.0 0.0 0.0 0.010 16 264 0 0 264 100.0 0.0 0.0 0.012 17 260 0 0 260 100.0 0.0 0.0 0.013 18 242 19 0 261 92.7 7.3 0.0 0.013 19 248 16 0 264 93.9 6.1 0.0 0.015 20 251 11 0 262 95.8 4.2 0.0 0.016 21 250 12 0 262 95.4 4.6 0.0 0.012 22 243 8 9 260 93.5 3.1 3.5 0.014 23 253 0 7 260 97.3 0.0 2.7 0.014 24 250 0 10 260 96.2 0.0 3.8 0.011 25 251 0 13 264 95.1 0.0 4.9 0.012 26 250 14 0 264 94.7 5.3 0.0 0.015 27 248 0 16 264 93.9 0.0 6.1 0.016

TABLE 2 Chemical composition of steel material Test C Si Mn Al Ni Cu Cr Li Mg Ce La No. (wt. %) (wt. %) (wt. %) (wt. %) (wt. %) (wt. %) (wt. %) (ppm) (ppm) (ppm) (ppm) 1 0.88 0.34 0.47 0.003 0.00 0.62 0.00 0.00 0.00 68 0 2 0.82 0.24 0.49 0.002 0.55 0.00 0.65 0.05 0.00 0 0 3 0.71 0.22 0.45 0.001 0.52 0.84 1.37 1.87 0.06 13 37 4 0.83 0.25 0.51 0.001 0.00 0.00 0.51 0.00 0.00 0 0 5 0.73 0.21 0.48 0.002 0.00 0.00 0.00 0.02 8.40 0 0 6 0.78 0.35 0.52 0.001 0.00 0.00 0.00 0.00 0.00 0 50 7 0.77 0.23 0.50 0.002 0.00 0.35 0.35 0.00 0.00 0 0 8 0.80 0.18 0.52 0.002 0.00 0.00 0.00 0.00 0.00 0 0 9 0.78 0.20 0.49 0.002 0.00 0.00 0.00 0.00 0.00 0 0 10 0.83 0.18 0.54 0.002 0.45 0.00 0.00 0.00 10.00 0 0 11 0.88 0.34 0.47 0.003 0.00 0.62 0.00 0.00 0.00 68 0 12 0.78 0.35 0.52 0.001 0.00 0.00 0.00 0.00 0.00 0 0 13 0.77 0.23 0.50 0.002 0.00 0.35 0.35 0.00 0.00 0 0 14 0.82 0.24 0.49 0.002 0.55 0.00 0.65 0.03 0.00 0 0 15 0.82 0.21 0.45 0.001 0.00 0.00 0.00 0.00 0.00 0 0 16 0.82 0.24 0.49 0.002 0.80 0.00 0.57 0.00 0.00 0 0 17 0.85 0.24 0.55 0.003 0.00 0.00 0.00 0.00 7.50 0 0 18 0.81 0.19 0.55 0.003 0.00 0.00 0.00 0.00 0.00 27 0 19 0.83 0.22 0.61 0.001 0.00 0.00 0.00 0.00 0.00 0 0 20 1.05 0.25 0.58 0.001 0.00 0.00 0.00 0.00 0.00 15 24 21 0.97 0.23 0.56 0.002 0.25 0.31 0.43 15.45 0.00 0 0 22 0.77 0.24 0.63 0.001 0.00 0.50 0.00 0.00 0.00 32 55 23 0.85 0.34 0.50 0.002 0.00 0.00 1.21 0.00 0.00 68 0 24 0.72 0.31 0.42 0.001 0.00 0.00 0.00 0.00 0.00 0 0 25 0.77 0.20 0.63 0.002 0.00 0.65 1.41 0.00 0.00 26 0 26 0.82 0.15 0.63 0.001 0.00 0.00 0.00 0.00 0.00 0 0 27 0.97 0.30 0.80 0.001 0.51 0.00 0.57 0.00 0.00 0 0

TABLE 3 Gas flow rate in Flow rate of Number of Al2O3- stirring of molten purging type inclusions with Number of steel in Ar gas to a long diameter of 20 μm breakages of Test secondary refining tundish or more steel wire per 10 t No. (Nm3/min/t) (Nm3/min/t) (number/1000 g) (number/10 t) 1 0.0010 0.08 2 1.9 2 0.0025 0.07 3 2.8 3 0.0037 0.10 5 3.9 4 0.0030 0.09 6 5.6 5 0.0005 0.04 9 7.8 6 0.0038 0.07 8 7.0 7 0.0009 0.05 5 6.1 8 0.0013 0.06 7 7.8 9 0.0009 0.10 6 13.5 10 0.0011 0.08 14 16.1 11 0.0003 0.08 15 15.4 12 0.0045 0.08 13 14.2 13 0.0002 0.08 17 16.8 14 0.0075 0.08 19 17.7 15 0.0011 0.12 6 5.0 16 0.0023 0.03 20 24.1 17 0.0025 0.02 25 28.0 18 0.0010 0.07 15 17.3 19 0.0025 0.07 15 13.6 20 0.0013 0.09 16 12.0 21 0.0030 0.08 13 12.7 22 0.0005 0.12 17 16.1 23 0.0010 0.08 13 12.2 24 0.0038 0.11 18 14.3 25 0.0009 0.08 18 18.0 26 0.0003 0.08 21 19.5 27 0.0070 0.08 25 20.0

These results suggest the following (No. below denotes the Test No. in Tables 1 to 3).

In No. 1 to 8, the conditions stipulated by the present invention were satisfied. Therefore, it is clear that the number of Al₂O₃ inclusions decreased, the number of wire breakages during wire drawing was small, and excellent drawability was attained. By contrast, in No. 9 to 27, the conditions stipulated by the present invention were not satisfied. As a result, the number of wire breakages during wire drawing was large and drawability degraded.

More specifically, in No. 9, 10, the concentration of P in the raw materials for a converter exceeded 0.02%. Therefore, excellent drawability could not be ensured. In No. 11 to 14, the mixing ratio of raw materials, P concentration, and purge Ar flow rate in the tundish were within the ranges stipulated by the present invention, but the flow rate of the stirring gas for the molten steel in secondary refining was outside the range stipulated by the present invention. As a result, drawability was degraded. In No. 15 to 17, the mixing ratio of main raw materials, P concentration, and flow rate of the stirring gas for the molten steel in secondary refining were within the ranges stipulated by the present invention, but the purge Ar flow rate in the tundish was outside the range stipulated by the present invention. As a result, drawability was degraded. In No. 18 to No. 25, P concentration of main raw materials, flow rate of the stirring gas for the molten steel in secondary refining and the purge Ar flow rate in the tundish were within the ranges stipulated by the present invention, but the mixing ratio of the main raw materials was outside the range stipulated by the present invention, therefore drawability was degraded. In No. 26, 27, the purge Ar flow rate in the tundish was within the range stipulated by the present invention, but the mixing ratio of raw materials and the flow rate of the stirring gas for the molten steel in secondary refining were outside the ranges stipulated by the present invention. As a result, drawability was degraded.

Embodiment 2 Evaluation of Fatigue Property

Fatigue property in the case of applying the steel wire rod with a diameter of 8.0 mm that was obtained in the above-described manner to a spring was evaluated according to the following items.

(Evaluation method) Nakamura-type rotating-bending fatigue test of a steel wire rod with a diameter of 8.0 mm

(Sample Preparation Method and Test Method)

A steel wire rod with a diameter of 8.0 mm was subjected sequentially to oil tempering, stress-relief annealing, shot peening, and secondary stress-relief annealing. Then, a fatigue test was performed under the following conditions by using a Nakamura-type rotating-bending fatigue machine and the fatigue property was evaluated by finding a wire breakage ratio.

(Fatigue Test Conditions)

Test piece length: 650 mm,

Number of test pieces: 30,

Test load: 95.8 kgf/mm² (940 MPa),

Rotating speed: 4,500 rpm,

Frequency of test stop: 2×10⁷,

Calculation formula for wire breakage ratio: Wire breakage ratio=(number of broken test pieces)/(total number of test pieces)×100(%)

The conditions relating to the main raw materials for a converter are shown in Table 4 below, the chemical compositions of the steel wire rods are shown in Table 5 below, and the results on drawability, together with the conditions of secondary refining, are shown in Table 6 below.

TABLE 4 Raw materials for converter Molten Cold Molten Cold iron Scrap P Test iron iron Scrap Total iron ratio ratio ratio concentration No. (t) (t) (t) (t) (wt. %) (wt. %) (wt. %) (wt. %) 28 263 0 0 263 100.0 0.0 0.0 0.010 29 258 0 0 258 100.0 0.0 0.0 0.013 30 261 0 2 263 99.2 0.0 0.8 0.011 31 252 0 5 257 98.1 0.0 1.9 0.011 32 250 5 5 260 96.2 1.9 1.9 0.015 33 260 3 0 263 98.9 1.1 0.0 0.014 34 255 5 0 260 98.1 1.9 0.0 0.017 35 256 7 0 263 97.3 2.7 0.0 0.016 36 251 10 0 261 96.2 3.8 0.0 0.015 37 255 5 0 260 98.1 1.9 0.0 0.022 38 261 0 0 261 100.0 0.0 0.0 0.026 39 260 0 0 260 100.0 0.0 0.0 0.007 40 260 0 4 264 98.5 0.0 1.5 0.010 41 260 4 2 266 97.7 1.5 0.8 0.012 42 260 0 0 260 100.0 0.0 0.0 0.012 43 264 0 0 264 100.0 0.0 0.0 0.012 44 249 10 0 259 96.1 3.9 0.0 0.013 45 251 13 0 264 95.1 4.9 0.0 0.013 46 245 15 0 260 94.2 5.8 0.0 0.013 47 234 20 0 254 92.1 7.9 0.0 0.011 48 243 9 5 257 94.6 3.5 1.9 0.015 49 247 4 11 262 94.3 1.5 4.2 0.014 50 253 0 10 263 96.2 0.0 3.8 0.013 51 245 0 14 259 94.6 0.0 5.4 0.011 52 245 0 16 261 93.9 0.0 6.1 0.011 53 245 0 16 261 93.9 0.0 6.1 0.011 54 245 19 0 264 92.8 7.2 0.0 0.014

TABLE 5 Chemical composition of steel material Test C Si Mn Al Ni Cu Cr Li Mg Ce La No. (wt. %) (wt. %) (wt. %) (wt. %) (wt. %) (wt. %) (wt. %) (ppm) (ppm) (ppm) (ppm) 28 0.70 1.99 0.75 0.003 0.00 0.00 1.15 0.00 0.00 5 0 29 0.71 1.80 0.77 0.002 0.00 0.00 0.00 0.00 0.00 0 94 30 0.58 1.45 0.55 0.002 0.00 0.00 0.00 0.00 6.50 0 0 31 0.62 1.85 0.85 0.002 0.00 0.00 0.00 0.00 0.00 0 0 32 0.67 1.93 0.77 0.001 0.00 0.00 0.00 0.19 0.00 0 0 33 0.70 2.02 0.82 0.001 0.33 0.00 1.21 0.00 0.05 0 0 34 0.65 1.51 0.61 0.001 0.00 0.21 0.00 0.00 0.00 0 5 35 0.72 1.99 0.75 0.003 0.92 0.84 1.37 0.32 0.20 52 81 36 0.68 1.87 0.77 0.002 0.00 0.00 0.00 0.00 0.00 73 78 37 0.70 1.80 0.81 0.001 0.00 0.00 0.00 0.00 0.00 0 0 38 0.65 1.96 0.77 0.002 0.00 0.00 0.56 0.00 14.00 0 0 39 0.66 1.50 0.80 0.003 0.00 0.00 0.62 0.00 0.00 0 0 40 0.68 1.85 0.91 0.001 0.00 0.00 0.63 0.23 0.00 0 0 41 0.67 2.06 0.84 0.002 0.35 0.00 0.35 0.00 0.00 0 0 42 0.69 1.45 0.81 0.002 0.00 0.00 0.00 0.00 0.00 0 0 43 0.62 1.49 0.69 0.002 0.00 0.00 0.00 0.00 0.00 0 0 44 0.59 1.67 0.78 0.003 0.00 0.00 0.00 0.00 0.00 0 0 45 0.68 1.91 0.88 0.001 0.00 0.00 0.75 0.00 0.00 0 0 46 0.68 1.91 0.88 0.001 0.00 0.31 0.75 0.00 0.00 0 55 47 0.59 1.46 0.81 0.003 0.00 0.00 0.00 0.00 0.00 0 98 48 0.61 1.47 0.68 0.002 0.00 0.00 0.00 0.00 0.00 0 0 49 0.68 1.99 0.76 0.001 0.00 0.00 0.86 0.52 0.06 0 0 50 0.68 1.91 0.88 0.001 0.34 0.00 1.25 0.00 0.00 0 0 51 0.59 1.46 0.81 0.003 0.00 0.00 0.00 0.00 0.00 0 0 52 0.62 1.74 0.74 0.002 0.00 0.48 1.29 0.30 0.00 44 0 53 0.62 1.74 0.74 0.002 0.00 0.00 0.35 0.00 0.00 0 0 54 0.70 2.01 0.87 0.002 0.00 0.00 0.42 0.22 0.00 0 0

TABLE 5 Chemical composition of steel material Test C Si Mn Al Ni Cu Cr Li Mg Ce La No. (wt. %) (wt. %) (wt. %) (wt. %) (wt. %) (wt. %) (wt. %) (ppm) (ppm) (ppm) (ppm) 28 0.70 1.99 0.75 0.003 0.00 0.00 1.15 0.00 0.00 5 0 29 0.71 1.80 0.77 0.002 0.00 0.00 0.00 0.00 0.00 0 94 30 0.58 1.45 0.55 0.002 0.00 0.00 0.00 0.00 6.50 0 0 31 0.62 1.85 0.85 0.002 0.00 0.00 0.00 0.00 0.00 0 0 32 0.67 1.93 0.77 0.001 0.00 0.00 0.00 0.19 0.00 0 0 33 0.70 2.02 0.82 0.001 0.33 0.00 1.21 0.00 0.05 0 0 34 0.65 1.51 0.61 0.001 0.00 0.21 0.00 0.00 0.00 0 5 35 0.72 1.99 0.75 0.003 0.92 0.84 1.37 0.32 0.20 52 81 36 0.68 1.87 0.77 0.002 0.00 0.00 0.00 0.00 0.00 73 78 37 0.70 1.80 0.81 0.001 0.00 0.00 0.00 0.00 0.00 0 0 38 0.65 1.96 0.77 0.002 0.00 0.00 0.56 0.00 14.00 0 0 39 0.66 1.50 0.80 0.003 0.00 0.00 0.62 0.00 0.00 0 0 40 0.68 1.85 0.91 0.001 0.00 0.00 0.63 0.23 0.00 0 0 41 0.67 2.06 0.84 0.002 0.35 0.00 0.35 0.00 0.00 0 0 42 0.69 1.45 0.81 0.002 0.00 0.00 0.00 0.00 0.00 0 0 43 0.62 1.49 0.69 0.002 0.00 0.00 0.00 0.00 0.00 0 0 44 0.59 1.67 0.78 0.003 0.00 0.00 0.00 0.00 0.00 0 0 45 0.68 1.91 0.88 0.001 0.00 0.00 0.75 0.00 0.00 0 0 46 0.68 1.91 0.88 0.001 0.00 0.31 0.75 0.00 0.00 0 55 47 0.59 1.46 0.81 0.003 0.00 0.00 0.00 0.00 0.00 0 98 48 0.61 1.47 0.68 0.002 0.00 0.00 0.00 0.00 0.00 0 0 49 0.68 1.99 0.76 0.001 0.00 0.00 0.86 0.52 0.06 0 0 50 0.68 1.91 0.88 0.001 0.34 0.00 1.25 0.00 0.00 0 0 51 0.59 1.46 0.81 0.003 0.00 0.00 0.00 0.00 0.00 0 0 52 0.62 1.74 0.74 0.002 0.00 0.48 1.29 0.30 0.00 44 0 53 0.62 1.74 0.74 0.002 0.00 0.00 0.35 0.00 0.00 0 0 54 0.70 2.01 0.87 0.002 0.00 0.00 0.42 0.22 0.00 0 0

TABLE 6 Gas flow rate in stirring of molten Flow rate of Number of Al2O3-type steel in purging Ar inclusions with a long Test secondary refining gas to tundish diameter of 20 μm or Wire breakage ratio No. (Nm3/min/t) (Nm3/min/t) more (number/1000 g) (%) 28 0.0010 0.08 3 17 29 0.0025 0.07 2 17 30 0.0037 0.10 4 27 31 0.0030 0.09 5 33 32 0.0005 0.04 9 37 33 0.0038 0.10 5 23 34 0.0009 0.05 4 27 35 0.0013 0.06 8 37 36 0.0012 0.10 8 43 37 0.0009 0.09 7 53 38 0.0007 0.10 16 63 39 0.0003 0.08 16 57 40 0.0045 0.08 12 53 41 0.0055 0.08 19 60 42 0.0010 0.12 4 27 43 0.0013 0.02 19 60 44 0.0018 0.01 25 70 45 0.0013 0.12 17 63 46 0.0013 0.13 17 63 47 0.0030 0.10 17 77 48 0.0010 0.09 20 80 49 0.0025 0.09 17 67 50 0.0013 0.09 17 63 51 0.0030 0.10 17 77 52 0.0005 0.09 22 77 53 0.0002 0.09 24 83 54 0.0080 0.09 25 83

These results suggest the following (No. below denotes the Test No. in Tables 4 to 6).

In No. 28 to 36, the conditions stipulated by the present invention were satisfied. Therefore, it is clear that the breakage ratio during the fatigue test was low and excellent fatigue property was attained. By contrast, in No. 37 to 54, the conditions stipulated by the present invention were not satisfied. As a result, the number of wire breakages during fatigue test was high and the fatigue property degraded.

More specifically, in No. 37, 38, the concentration of P in the raw materials for a converter exceeded 0.02%. Therefore, excellent fatigue property could not be ensured. In No. 39 to 41, the mixing ratio of raw materials, P concentration, and purge Ar flow rate in the tundish were within the ranges stipulated by the present invention, but the flow rate of the stirring gas for the molten steel in secondary refining was outside the range stipulated by the present invention. As a result, the fatigue property was degraded. In No. 42 to 44, the mixing ratio of raw materials, P concentration, and flow rate of the stirring gas for the molten steel in secondary refining were within the ranges stipulated by the present invention, but the purge Ar flow rate in the tundish was outside the range stipulated by the present invention. As a result, the fatigue property was degraded. In No. 45 to 52, the P concentration of the main raw materials, flow rate of the stirring gas for the molten steel in secondary refining, and purge Ar flow rate in the tundish were within the ranges stipulated by the present invention, but the mixing ratio of raw materials was outside the range stipulated by the present invention. As a result, the fatigue property was degraded. In No. 53, 54, the purge Ar flow rate in the tundish was within the range stipulated by the present invention, but the mixing ratio of raw materials and the flow rate of the stirring gas for the molten steel in secondary refining were outside the ranges stipulated by the present invention. As a result, drawability was degraded. 

1. A method for manufacturing steel for high-carbon steel wire rod that excels in drawability and fatigue property, characterized in that converter blowing is performed by taking molten iron, cold iron, and steel scrap as main raw materials to be charged into a converter, the ratio of these components based on all the main raw materials being such that the molten iron takes 96 to 100% (means wt. %, same hereinbelow), the cold iron takes 4% or less, and the steel scrap takes 2% or less, and by setting an average P concentration in all the main raw materials to 0.02% or less, and operations are conducted such that a flow rate of gas for stirring molten steel during secondary refining after completion of the converter blowing is set to 0.0005 Nm³/min or more to 0.004 Nm³/min or less per 1 t of molten steel, and then a flow rate of Ar used to purge the inside of a tundish in continuous casting is set to 0.04 Nm³/min or more to 0.10 Nm³/min or less per 1 t of molten steel within the tundish.
 2. The manufacturing method according to claim 1, wherein a chemical composition of the steel for steel wire rod comprises C: 0.4 to 1.3%, Si: 0.1 to 2.5%, Mn: 0.2 to 1.0%, and Al: 0.003% or less with Fe and unavoidable impurities as the balance.
 3. The manufacturing method according to claim 1, wherein the steel for steel wire rod further comprises as another element at least one element selected from the group consisting of Ni: 0.05 to 1%, Cu: 0.05 to 1%, and Cr: 0.05 to 1.5%.
 4. The manufacturing method according to claim 1, wherein the steel for steel wire rod further comprises as another element at least one element selected from the group consisting of Li: 0.02 to ppm, Mg: 0.02 to 20 ppm, Ce: 3 to 100 ppm, and La: 3 to 100 ppm. 